The penetration resistance of the spade-like members, which can be broken down into a front or tip resistance and a lateral sliding resistance, varies with the nature of the layers of ground passed through, particularly according to whether the layers have a sand or clay base.
This is the reason for the disadvantages inherent in the prior art solution of the problem, which is for the spade-like members to have a continuous horizontal edge, for, to an extent depending upon the nature of the ground penetrated, it does not always prove possible for the spade-like members to penetrate stably and fully. There is a risk of partial penetration with voids below the base or else an excessive weight of ballast may have to be used.
It has been suggested that the spade-like members should have minor undulations, but the result is unsatisfactory.